1. Field of the Invention
The present invention relates to a method to manufacture a semiconductor optical device, in particular, the invention relates to a method to manufacture a semiconductor light-emitting device with an diffraction grating.
2. Related Prior Arts
Photonics technology letters has disclosed a distributed feedback laser diode (hereafter denoted as DFB-LD) in a volume 16, pages from 2415 to 2417, published in 2004. The DFB-LD disclosed therein includes an active layer made of AlGaInAs and a diffraction grating between the active layer and an n-type InP substrate.
The relaxation frequency of a semiconductor LD strongly affects high-frequency performance of the LD. The DFB-LD has been requested in the performance thereof to enhance the relaxation frequency as high as possible in an operable temperature range. It is effective for enhancing the relaxation frequency to set a difference between the Bragg diffraction wavelength, which is determined by the pitch of the diffraction grating implemented in the LD and decides the oscillation wavelength of the DFB-LD, and a peak wavelength of the optical gain in the active layer within a characteristic range. This difference is often called as the detuning amount.
A DFB-LD, where the diffraction grating thereof is formed after the growth of the active layer, may adjust the pitch of the diffraction grating based on characteristics of the active layer, which may eliminate the fluctuation or the variation of the oscillation wavelength of the DFB-LD caused by the instability of the growth of the active layer.
However, the diffraction grating of the DFB-LD is often implemented between the n-type InP and the active layer to improve the temperature dependence and the high frequency performance of the DFB-LD. In such a DFB-LD, the diffraction grating is foamed before the growth of the active layer, and the pitch of the diffraction grating may be determined based on an estimation of the characteristic of the active layer grown subsequent to the diffraction grating. Therefore, when the characteristic of the active layer, for instance, the composition thereof, becomes out of the estimation; the detuning amount shifts from the designed value. The peak wavelength of the optical gain in the active layer generally shows a variation of a few nano-meters. The DFB-LD with the diffraction grating formed in advance to the formation of the active layer inherently has a variation of the detuning amount around a few nano-meters.
The present invention is to provide a method to manufacture an DFB-LD with the diffraction grating formed in advance to the formation of the active layer, in which the DFB-LD according to the present invention shows an improved high-frequency performance.